Seismic StratigraphyEPS 444

Dr. Faisal A. Alqahtani2011

email:falqahtani@kau.edu.sa

Seismic techniques• Seismic techniques generally involve measuring the travel time of certain types of seismic energy

from surficial shots (i.e. an explosion or weight drop) through the subsurface to arrays of ground motion sensors or geophones.

• In the subsurface, seismic energy travels in waves that spread out as hemispherical wavefronts (i.e. the three dimensional version of the ring of ripples from a pebble dropped into a pond).

• The energy arriving at a geophone is described as having traveled a ray path perpendicular to the wavefront (i.e. a line drawn from the spot where the pebble was dropped to a point on the ripple). In the subsurface, seismic energy is refracted (i.e. bent) and/or reflected at interfaces between materials with different seismic velocities (i.e. different densities).

• The refraction and reflection of seismic energy at density contrasts follows exactly the same laws that govern the refraction and reflection of light through prisms.

• Note that for each seismic ray that strikes a density contrast a portion of the energy is refracted into the underlying layer, and the remainder is reflected at the angle of incidence.

• The reflection and refraction of seismic energy at each subsurface density contrast, and the generation of surface waves (or ground roll), and the sound (i.e. the air coupled wave or air blast) at the ground surface all combine to produce a long and complicated sequence of ground motion at geophones near a shot point.

• The ground motion produced by a shot is typically recorded as a wiggle trace for each geophone

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Seismic acquisition offshore• An air gun towed behind the

survey ship transmits sound waves through the water column and into the subsurface

• Changes in rock type or fluid content reflect the sound waves towards the surface

• Receivers towed behind the vessel record how long it takes for the sound waves to return to the surface

• Sound waves reflected by different boundaries arrive at different times.

• The same principles apply to onshore acquisition

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• Onshore seismic acquisition requires an energy input from a “thumper” truck. Geophones arrayed in a line behind the truck record the returning seismic signal.

Sub-horizontal bedsUnconformityDipping beds

Seismic acquisition onshore (1)

Geophones (receivers)

Vibrator (source)

Seismic acquisition onshore (2)

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Lithology changeAngular unconformityLithology change

• Seismic horizons represent changes in density and allow the subsurface geology to be interpreted.

Types of Waves• Seismic Wave– Body Waves

• Primary or p-wave – Compression wave

• Secondary or s-wave – Transverse wave

– Surface• Love wave• Rayleigh wave

Seismic Wave

• Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

• There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves.

Body Waves

• P Waves (compression wave)• The first kind of body wave is the P wave or primary wave. This is the

fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock and it moves through just like sound waves push and pull the air.

Stop and Think

• Have you ever heard a big clap of thunder and heard the windows rattle at the same time?

• The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Usually we only feel the bump and rattle of these waves.

Body Waves• S wave (transverse wave)• The second type of body wave is the S wave or secondary

wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock. This wave moves rock up and down, or side-to-side.

Surface Waves • Love Waves • The first kind of surface wave is called a Love wave, named

after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.

Surface Waves• Rayleigh Waves• The other kind of surface wave is the Rayleigh wave, named for John William

Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Seismic Refraction

• Seismic refraction involves measuring the travel time of the component of seismic energy which travels down to the top of rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface as a head wave along a wave front similar to the bow wake of a ship (see Seismic Refraction Geometry below). The shock waves which return from the top of rock are refracted waves, and for geophones at a distance from the shot point, always represent the first arrival of seismic energy.

• Seismic refraction is generally applicable only where the seismic velocities of layers increase with depth. Therefore, where higher velocity (e.g. clay) layers may overlie lower velocity (e.g. sand or gravel) layers, seismic refraction may yield incorrect results. In addition, since seismic refraction requires geophone arrays with lengths of approximately 4 to 5 times the depth to the density contrast of interest (e.g. the top of bedrock), seismic refraction is commonly limited (as a matter of practicality) to mapping layers only where they occur at depths less than 100 feet. Greater depths are possible, but the required array lengths may exceed site dimensions, and the shot energy required to transmit seismic arrivals for the required distances may necessitate the use of very large explosive charges. In addition, the lateral resolution of seismic refraction data degrades with increasing array length since the path that a seismic first arrival travels may migrate laterally (i.e. in three dimensions) off of the trace of the desired (two dimensional) seismic profile.

• Recent advances in inversion of seismic refraction data have made it possible to image relatively small, non-stratigraphic targets such as foundation elements, and to perform refraction profiling in the presence of localized low velocity zones such as incipient sinkholes.

• Seismic reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts (see Seismic Refraction Geometry below). Reflected seismic energy is never a first arrival, and therefore must be identified in a generally complex set of overlapping seismic arrivals - generally by collecting and filtering multi-fold or highly redundant data from numerous shot points per geophone placement. Therefore, the field and processing time for a given lineal footage of seismic reflection survey are much greater than for seismic refraction. However, seismic reflection can be performed in the presence of low velocity zones or velocity inversions, generally has lateral resolution vastly superior to seismic refraction, and can delineate very deep density contrasts with much less shot energy and shorter line lengths than would be required for a comparable refraction survey depth.

• The main limitations to seismic reflection are its higher cost than refraction (for sites where either technique could be applied), and its practical limitation to depths generally greater than approximately 50 feet. At depths less than approximately 50 feet, reflections from subsurface density contrasts arrive at geophones at nearly the same time as the much higher amplitude ground roll (surface waves) and air blast (i.e. the sound of the shot). Reflections from greater depths arrive at geophones after the ground roll and air blast have passed, making these deeper targets easier to detect and delineate.

• Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) where the inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land.

Seismic Refraction

Seismic method comparisonRefraction Reflection

Typical Targets Near-horizontal density contrasts at depths less than ~100 feet

Horizontal to dipping density contrasts, and laterally restricted targets such as cavities or tunnels at depths greater than ~50 feet

Required Site Conditions Accessible dimensions greater than ~5x the depth of interest; unpaved greatly preferred

None

Vertical Resolution 10 to 20 percent of depth 5 to 10 percent of depth

Lateral Resolution ~1/2 the geophone spacing ~1/2 the geophone spacing

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• Wiggle trace to CDP gather• Normal move out correction• Stacking• What is a reflector?

Seismic processing

Wiggle trace to CDP gather

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Graphs of intensity of sound as received by the recorders

Graphs of intensity for one location collected into groups and shown in a sequence.

Wiggle traces

CDP gather

Normal move out correction

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Data for one point from different signals to different receivers 1. More time needed to reach distant receivers so the data look like a curve. 2. Correcting for normal move out restores the curve to a near horizontal display.

Change in lithology from mud to sand so sound is reflected back to surface

Sound receiversR3 R2 R1

Sound wave in

CDP

CMP

Fast

est

Slowes

t

Sound sources S1 S2 S3

Original CDP gather …

corrected for normal move out

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Wave

reflec

ted

Stacking

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Next, take all the sound traces for that one place and stack them on top of each other

First, gather sound data for one location and correct for delayed arrival (normal move out)

Finally, place stacks for adjacent locations side by side to produce a seismic line

What is a reflector?

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There are many reflectors on a seismic section. Major changes in properties usually produce strong, continuous reflectors as shown by the arrow.

A seismic reflector is a boundary between beds with different properties. There may be a change of lithology or fluid fill from Bed 1 to Bed 2. These property changes cause some sound waves to be reflected towards the surface.

Bed 1

Bed 2

Incoming ray Reflected ra

y

Refracted ray

lower velocityhigher velocity

energy source

signal receiver

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Understanding the data

• Common Depth Points (CDPs)• Floating datum• Two way time (TWT)• Time versus depth

Common Depth Points

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CDPs are defined as ‘the common reflecting point at depth on a reflector or the halfway point when a wave travels from a source to a reflector to a receiver’.

Common midpoint above

CDP

Change in lithology = reflecting horizon

Common reflecting point or common depth point (CDP)

Sound sources S1 S2 S3

Sound receiversR3 R2 R1

Sound wave in Sound

wav

e

reflec

ted

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The floating datum line represents travel time between the recording surface and the zero line (generally sea level). This travel time depends on rock type, how weathered the rock is, and other factors.

The topographic elevation is the height above sea level of the surface along which the seismic data were acquired.

Floating datum

Two way time (TWT)

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0.25 seconds

Two way time (TWT) indicates the time required for the seismic wave to travel from a source to some point below the surface and back up to a receiver.

In this example the TWT is 0.5 seconds.

0.25 seconds

0

0.5

TWT

seco

nds

surface

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Time versus depth

0.58 sec

m

1865

926

288

926 m

• Two way time (TWT) does not equate directly to depth• Depth of a specific reflector can be determined using

boreholes• For example, 926 m depth = 0.58 sec. TWT

Hw 1 & quiz next week• Rayan: Data types (seismic , well logs, etc..)

• Thamer: History of Sequence Stratigraphy (sequence boundary, parasequance, etc.)

• Saeed: Lowstand system tract

• Mohammed: Transgrassive system tract

• Rakan: Highstand system tract

• Talal: Condensed section

• Moaid: 3D seimsic data